Nitride stabilized core/shell nanoparticles

ABSTRACT

Nitride stabilized metal nanoparticles and methods for their manufacture are disclosed. In one embodiment the metal nanoparticles have a continuous and nonporous noble metal shell with a nitride-stabilized non-noble metal core. The nitride-stabilized core provides a stabilizing effect under high oxidizing conditions suppressing the noble metal dissolution during potential cycling.

CROSS-REFERENCE TO A RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 14/554,583, filed Nov. 26, 2014, which claims the benefit under35 U.S.C. 119(e) of U.S. Provisional Application No. 61/909,743 filed onNov. 27, 2013, the disclosures of which are hereby incorporated byreference in their entirety.

STATEMENT OF GOVERNMENT RIGHTS

This invention was made with Government support under contract numbersDE-AC02-98CH10886 and DE-SC0012704, awarded by the U.S. Department ofEnergy. The Government may have certain rights in this invention.

FIELD OF THE INVENTION

This disclosure relates to the field of electrochemical catalysts andthe methods for their manufacture and use. In particular, the disclosurerelates to nanometer-scale electrocatalysts, primarily platinum based,with a nitrogen stabilized core, and a method of using the synthesizedelectrocatalysts for polymer electrolyte fuel cells.

BACKGROUND

Metals such as platinum (Pt), palladium (Pd), ruthenium (Ru), andrelated alloys are known to be excellent catalysts. When incorporated inelectrodes of an electrochemical device, such as a fuel cell, thesematerials function as electrocatalysts as the materials accelerateelectrochemical reactions at electrode surfaces, yet are not themselvesconsumed by the overall reaction. Although noble metals have been shownto be some of the best electrocatalysts, their successful implementationin commercially available energy conversion devices may be limited dueto high cost and scarcity. Noble metal catalysts may also be susceptibleto carbon monoxide (CO) poisoning, poor stability under cyclic loading,and providing relatively slow conversion kinetics in oxygen reductionreactions (ORR).

A variety of approaches have been employed in attempts to address theseissues. One approach involves increasing the overall surface areaavailable for reaction by forming metal particles with nanometer-scaledimensions. Loading of more expensive noble metals such as Pt has beenreduced by forming nanoparticles from alloys comprised of Pt and alow-cost component. Further improvements have been attained by formingcore-shell nanoparticles, in which a core particle is coated with ashell of a different material that functions as the electrocatalyst. Thecore is usually a low-cost material which is easily fabricated whereasthe shell comprises a more catalytically active noble metal. An exampleis provided by U.S. Pat. No. 6,670,301 to Adzic, et al., which disclosesa process for depositing a thin film of Pt on dispersed Ru nanoparticlessupported by carbon substrates. Another example is U.S. Pat. No.7,691,780 to Adzic, et al. which discloses platinum- and platinumalloy-coated palladium and palladium alloy nanoparticle cores. Each ofthe aforementioned U.S. Patents is incorporated by reference in itsentirety as if fully set forth in this specification.

One approach for synthesizing core-shell particles with reduced noblemetal loading and enhanced activity levels involves the use ofelectrochemical routes, which provide atomic-level control over theformation of uniform and conformal ultrathin coatings of the desiredmaterial on a large number of three-dimensional nanoparticles. One suchmethod involves the initial deposition of an atomic monolayer of a metalsuch as copper (Cu) onto a plurality of nanoparticles by underpotentialdeposition (UPD). This is followed by galvanic displacement of theunderlying Cu atoms by a noble metal such as Pt as disclosed, forexample, in U.S. Pat. No. 7,704,918 to Adzic, et al. Another methodinvolves hydrogen adsorption-induced deposition of a monolayer of metalatoms on noble metal particles as described, for example, by U.S. Pat.No. 7,507,495 to Wang, et al. Each of the aforementioned U.S. Patents isincorporated by reference in its entirety as if fully set forth in thisspecification.

Core-shell particles having a core comprised of one or more non-noblemetals may show gradual dissolution of the non-noble metal componentover time. Exposure of the core to the corrosive environment typicallypresent in energy conversion devices such as a proton exchange membranefuel cell (PEMFC) due to, for example, an incomplete protective shelllayer may result in gradual erosion of the non-noble metal componentswhich leads to the loss of structural integrity of the particles. Withcontinued operation, reduction in structural integrity may reduce thecatalytic activity of the electro catalyst and cause damage to theelectrolyte membranes contained within a typical energy conversiondevice, thereby reducing its charge storage and energy conversioncapabilities.

There is therefore a continuing need to develop catalysts with a stillhigher catalytic activity in combination with ever-lower loading ofprecious metals, enhanced durability, and long-term stability. Suchcatalysts should also be capable of being manufactured by large-scaleand cost-effective processes suitable for commercial production andincorporation in conventional energy production devices.

SUMMARY

In view of the above-described problems, needs, and goals, a novelelectrocatalytic nanoparticle with a nitride stabilized non-noble metalor non-noble metal alloy core is provided that has utility in, amongothers, electrochemical reactions, such as fuel cells, for example,polymer electrolyte membrane fuel cells (PEMFCs). In one embodiment, thenanoparticle has nano-sized external dimensions and is characterized bya continuous and nonporous shell with a nitride stabilized core. In aparticular embodiment the structure of the nitride stabilized core issuch that it enhances the shell's oxygen-reduction reaction (ORR)activity and provides a stabilizing effect under highly oxidizingconditions found in fuel cells, thus, suppressing dissolution duringpotential cycling.

In another embodiment the disclosed nanoparticle is manufactured by amethod which, in its most basic form, involves preparing a solutioncontaining an organic solvent, salts of a noble metal and a non-nobletransition metal, and a carbon powder; removal of the organic solvent byevaporation to form dried powders; followed by thermal annealing inammonia (NH₃) gas of the dried powders to form a metal nitride core anda thin noble metal shell. The manufacturing process is simple andcost-effective, providing nitride stabilized nanoparticles with stillhigher catalytic activities and improved durability in combination withminimal loading of precious materials as compared with catalystscurrently in use.

The solution may comprise a soluble salt of Ni and a soluble salt of Ptin an organic solution. The soluble salt of Ni may be, for example,Ni(acac)₂. The soluble salt of Pt may be, for example, Pt(acac)₂.Formation of the core/shell nanoparticles may be accomplished byannealing the dried powder at 400 to 700° C. under NH₃ to form a corecomprising nickel nitride Ni_(x)N (x=3 or 4). In yet another embodimentthe salt of the noble metal of the formation of shell may be Pd(acac)₂and is used in combination with a Ni salt to form nitride stabilizedNi-Pd core-shell nanoparticles. In yet another embodiment, the salt ofthe noble metal of the formation of shell may be Pd(acac)₂ andAu(CH₃)₂(acac) and is used in combination with a Ni salt to form nitridestabilized Ni-PdAu core-shell nanoparticles. In yet another embodimentthe salt of the noble metal of the formation of shell may be Ir(acac)₃and is used in combination with a Ni salt to form nitride stabilizedNi—Ir core-shell nanoparticles.

In one embodiment, the nanoparticle cores are comprised of a nitride ofa single non-noble transition metal, but may comprise a plurality ofelements or components. When more than one transition metal is used, thenanoparticle alloy may be a homogeneous solid solution, but it may alsohave compositional nonuniformities. The nitride of the non-nobletransition metal may be at least one of nickel (Ni) nitride, cobalt (Co)nitride, iron (Fe) nitride, copper (Cu) nitride, tungsten (W) and/ortheir alloys. In an embodiment, the nitride is nickel nitride (Ni₄N).The nanoparticle cores provide a template for deposition of one or aplurality of noble metals on core surfaces and also provide a durablecore for forming the noble metal shells.

In another embodiment, the material constituting the shell layer is anoble metal, and in another embodiment the shell is a noble metalalloyed with one or more transition metals, including other noblemetals. In an embodiment, the composition of the shell is homogeneous.However, the composition may, in other embodiments, be nonuniform. Thenoble metal shell may be comprised of at least one of palladium (Pd),iridium (Ir), rhenium (Re), ruthenium (Ru), rhodium (Rh), osmium (Os),gold (Au), and platinum (Pt), either alone or as an alloy. In anembodiment the shell is comprised of Pt. In yet another embodiment theshell is comprised of Pd or a Pd/Au alloy.

The disclosed core-shell nanoparticles have a continuous and nonporousexternal surface with a nitride stabilized core. In one embodiment thenanoparticles are substantially spherical with an external diameter ofless than 20 nm and a shell thickness of between 0.5 and 3 nm or,alternatively, a shell wall thickness of 1 to 12 atomic layers. Inanother embodiment, the external diameter of the nanoparticles isbetween 2 nm and 10 nm with a shell wall thickness of 2 to 8 atomiclayers. In yet another embodiment the nanoparticles have an averageexternal diameter of 3.5 nm, and a shell wall thickness of 2 to 4 atomiclayers. In one embodiment, the core of the nanoparticle has at least 2wt. % metal nitride. In another embodiment, the core of the nanoparticlehas at least 5 wt. % metal nitride. In yet another embodiment, the coreof the nanoparticle has at least 20 wt. % metal nitride. Thenanoparticles may be made of Pt shell and Ni₄N core, but may also bemade of Ir, Pd or a Pd/Au alloy shell and the Ni₄N core. In yet anotherembodiment the nanoparticles are made of Ni₄N core, an Ir, Pd or a Pd/Aualloy first shell which is covered with one or two monolayers of Ptshell. In an embodiment, the nanoparticle has a platinum (Pt) shellhaving a shell wall thickness of 2 to 4 atomic layers and nickel (Ni)core made with at least 5 wt. % nickel (Ni) nitride.

Nitride stabilized nanoparticles are particularly advantageous whenincorporated into one or more electrodes of an energy conversion device.The structure of such a device is designed with at least a firstelectrode, a conducting electrolyte, and a second electrode, in whichone or more of the first or second electrodes comprises metalnanoparticles. These nanoparticles have a continuous and nonporous shellwith a nitride stabilized metal core. It is shown herein that thenitrogen and the nanoparticle structure increase the catalytic activityof the noble metal shell and provide a stabilizing effect under highlyoxidizing conditions that suppresses dissolution during potentialcycling. In an embodiment, the nitride stabilized nanoparticlesincorporated into an energy conversion device are comprised of Pt andhave an external diameter of 3 nm to 20 nm with a shell wall thicknessof 2 to 4 atomic monolayers.

In present embodiments, the production of nitride stabilizednanoparticles permits a reduction in loading of precious materials whilesimultaneously enhancing the catalytically active surface area andimproving stability and durability of the resulting energy conversiondevices. The use of nitride stabilized nanoparticles as electrocatalystsfacilitates more efficient, durable, and cost-effective electrochemicalenergy conversion in devices such as fuel cells and metal-air batteries.The use of Pt-based nitride stabilized nanoparticles may also providesimilar advantages when used as a catalyst for oxidation of smallorganic molecules such as methanol and ethanol, where weakening Ptreactivity can enhance the catalyst's tolerance to poisoningintermediates or for hydrogenation reactions in producing renewablefuels.

These and other characteristics of the nitride stabilized nanoparticles,a method of synthesis/manufacture thereof, and a method of use willbecome more apparent from the following description and illustrativeembodiments which are described in detail with reference to theaccompanying drawings. Similar elements in each figure are designated bylike reference numbers and, hence, subsequent detailed descriptionsthereof may be omitted for brevity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a single platinum based (Pt)nanoparticle.

FIG. 2 is a schematic illustration of a single core/shell nanoparticlewith platinum (Pt) shell, and nickel/nickel nitride core.

FIG. 3a is high-angle annular dark-field image (HAADF) obtained fromscanning transmission electron microscope (STEM) of PtNiN core-shellnanoparticles with a sample size of 300.

FIG. 3b is a bar chart showing particle size distribution of PtNiNcore-shell nanoparticles with a sample size of 300.

FIG. 4a is a HAADF-STEM image of a PtNiN core-shell nanoparticle withits corresponding two-dimensional electron energy loss spectroscopy(EELS) mapping of Pt M and Ni L signals.

FIG. 4b is a STEM image of PtNiN core-shell nanoparticles.

FIG. 4c is a plot showing EELS line-scan profiles of Pt and Ni in asingle nanoparticle along with schematic representation of a singlePtNiN nanoparticle of FIG. 2.

FIG. 5a is a plot of a synchrotron X-ray diffraction (XRD) pattern forPtNiN catalyst showing Pt and Ni₄N phases. The lines next to the x-axisdenote Ni₃N phase.

FIG. 5b is a plot showing in situ normalized X-ray absorption near edgestructure (XANES) of Ni K edge for PtNiN nanoparticles at variouspotentials.

FIG. 5c is a plot showing comparison of the change of the Pt adsorptionedge peaks of the XANES spectra for PtNiN/C and Pt/C as a function ofpotential obtained in 1 M HClO₄.

FIG. 6 is a plot showing synchrotron XRD diffraction for PtNinanoparticles annealed in N₂ at 250° C. and in NH₃ at 510° C. The XRDanalyses confirm that Ni in the catalyst is nitrided and forms a richNi₄N species.

FIG. 7a is a plot showing CV curves for Pt/C (E-TEK) and PtNiNcore-shell nanoparticles in 0.1M HClO₄ acid at a scan rate of 20 mV/s.For Pt/C and PtNiN core-shell nanoparticles the Pt loading was 7.65 and7.84 g/cm², respectively.

FIG. 7b is a plot showing the ORR polarization curves for PtNiNcore-shell nanoparticles in 0.1 M HClO₄ acid at a scan rate of 10 mV/sat various rpm. Inset shows the Koutecky-Levich plot at 0.9 and 0.85V.

FIG. 8 is a plot of in situ XANES of Pt L3 edge for PtNiNelectrocatalyst at various potentials.

FIG. 9a is a plot showing polarization curves for ORR of Pt/C andPtNiN/C nanoparticles on an RDE electrode. Pt loading for PtNiN/C andPt/C were 7.84 and 7.65 μg/cm² respectively. The inset shows a bardiagram of mass and specific activities for PtNiN/C and Pt/C catalystson the RDE electrode.

FIG. 9b is a plot showing ORR polarization and voltammetry (inset)curves of PtNiN core-shell nanoparticles before and after 35000 cycletest between 0.6 and 1.05 V in 0.1 M HClO₄.

FIGS. 10a-10b are HAADF images of representative nanoparticles (a) and(b) after potential cycling along with its respective 2-D EELS mappingof Pt M edge (dark) and Ni L edge (light).

FIG. 11a is a low-mag HAADF-STEM image of PtNiN core-shell nanoparticlesafter 35000 potential cycles.

FIG. 11b is a high resolution HAADF image of PtNiN core-shellnanoparticles after 35000 potential cycles.

FIG. 11c is a plot showing EELS line-scan profiles to rationalize thedistribution of Pt and Ni components in a single representativenanoparticle from FIG. 11b after cycling. The results indicate thatPtNiN nanoparticles stay intact without losing their core-shellstructure after potential cycling.

FIG. 12 is a schematic of a sphere-like Pt_(2ML)/Ni₄N nanoparticle modelwith ˜1.7 nm used for DFT calculations.

FIG. 13a-13b are plots showing calculated binding energy of oxygen(BE-O) as a function of (a) surface strain and (b) d-band center. “1ML”and “2ML” are one monolayer and two monolayers, respectively. The dashedboxes are for the two monolayers.

FIG. 14a is a plot showing comparison of surface strain versus predictedbinding energy of oxygen (BE-O) on the Pt_(2ML)Ni₄N and Pt nanoparticlemodels with 1.7 nm. The more negative strain corresponds to furthercompression. An atomic oxygen was placed at a fcc active site on the(111) plane to predict the BE-Os.

FIG. 14b is a plot showing Pt specific activity against BE-O on PtNiN/Cand Pt/C.

FIG. 14c is a schematic of the inner Pt diffusion process to thedefective sites at the vertex during cycling in the electrolyte. Forclarity, defects are marked with a light shade of gray and the inner Ptatoms involved in diffusion are marked with a dark shade.

FIG. 15a is a TEM image of PtNiN nanoparticles on carbon-support.

FIG. 15b is a TEM image of PtNiN nanoparticles on carbon-support.

FIG. 15c is a TEM image of PtNiN nanoparticles on carbon-support.

FIG. 16 is a plot showing ORR polarization curves of carbon-supportedPtNiN nanoparticles before and after 10 k, 20 k, 30 k cycles.

FIG. 17 is a plot showing changes in mass activity (MA) and specificactivity (SA) as a function of the number of cycles in carbon-supportedPtNiN nanoparticles.

FIG. 18 is a plot showing changes in half-wave potential (E1/2) andelectrochemical surface area (ECSA) as a function of the number ofcycles in carbon-supported PtNiN nanoparticles.

FIG. 19a is a plot showing H₂/air polarization curves from MEA tests ofPtNiN/C and commercial Pt/C catalysts at a temperature of 80° C. at aback pressure of 247 kPa. Pt loadings: 0.065 mg/cm² for PtNiN/C and0.070 mg/cm² for Pt/C.

FIG. 19b is a plot showing H₂/air polarization curves from MEA tests ofPtNiN/C and commercial Pt/C catalysts at a temperature of 80° C. at aback pressure of 247 kPa. Pt loadings: 0.065 mg/cm² for PtNiN/C and0.070 mg/cm² for Pt/C.

FIG. 20a is a plot showing H₂/air polarization curves from MEA tests ofPtNiN/C before and after 10 k and 30 k cycles (one cycle: 0.6 V (3 s)and 0.95 V (3 s)) at a temperature of 80° C. at a back pressure of 247kPa. Pt loadings: 0.052 mg/cm².

FIG. 20b is a plot showing H₂/air polarization curves from MEA tests ofcommercial Pt/C catalysts before and after 10 k and 30 k cycles (onecycle: 0.6 V (3 s) and 0.95 V (3 s)) at a temperature of 80° C. at aback pressure of 247 kPa. Pt loadings: 0.052 mg/cm².

FIG. 21 is a schematic of a PEMFC having the electrodes with thedisclosed nitride stabilized nanoparticles.

DETAILED DESCRIPTION

Previous approaches to producing catalyst particles with a highercatalytic activity and reduced loading of costly precious metals havetypically involved the use of one or more components which aresusceptible to corrosion in alkaline or acidic environments. Over time,the gradual loss of these elements and their subsequent buildup in othercritical components present within the energy conversion device, e.g.,an electrolyte membrane, reduces both the activity level of the catalystparticles and the overall efficiency of the device.

These and other issues are addressed by embodiments disclosed herein inwhich electrocatalytic nanoparticles have a corrosion-resistant coreexhibiting a heightened catalytic activity and improved durability havebeen developed. It is believed that the enhanced activity and durabilityis attributable at least partly to geometric and electronic effects, inwhich the presence of a nitride within the non-noble metal coresuppresses core dissolution during potential cycling and reduces latticecontraction leading to an up-shifted noble metal d-band center. Whilenot wishing to be bound by any particular theory, the analysis describedherein reveals that nitride-induced contraction strengthens oxygenbinding at nanoparticle surfaces compared to a non-noble metal corealone, yet increases lattice contraction leading to a down-shifted noblemetal d-band center compared to the noble metal alone.

It is envisioned that one or more metals, as well as mixtures or alloysof these metals may be used as the material constituting the core and/orshell material without deviating from the spirit and scope of thepresent invention. Throughout this specification, the nitride stabilizednanoparticles and processes for their manufacture will be describedusing one or more metals due to the advantages provided by their use aselectrocatalysts and/or catalysts in general.

I. Nanoparticle Core

Initially, nanoparticle cores of a suitable metal or metal alloy areprepared using any technique that is well-known in the art. It is to beunderstood, however, that the present embodiments are not limited tonanoparticle cores that are comprised of a single element or materialthroughout, and the nanoparticle cores may also include nanoparticlealloys. A nanoparticle alloy is defined as a particle formed from acomplete solid solution of two or more elemental metals. However, suchnanoparticle alloys are not limited to homogeneous solid solutions, butmay also be inhomogeneous or heterogeneous. That is, the nanoparticlealloy may not have an even concentration distribution of each elementthroughout the nanoparticle itself. There may be precipitated phases,immiscible solid solutions, concentration nonuniformities, and somedegree of surface segregation.

The nanoparticle cores may be spherical or spheroidal with a sizeranging from 2 nm to 100 nm along at least one of three orthogonal axes,and are thus nanometer-scale particles or nanoparticles. It is to beunderstood, however, that the particles may take on any shape orstructure which includes, but is not limited to branching, conical,pyramidal, cubical, cylindrical, mesh, fiber, cuboctahedral,icosahedral, and tubular nanoparticles. The nanoparticles may beagglomerated or dispersed, formed into ordered arrays, fabricated intoan interconnected mesh structure, either formed on a supporting mediumor suspended in a solution, and may have even or uneven sizedistributions. The particle shape and size may be configured to maximizesurface catalytic activity. In an embodiment the nanoparticle cores haveexternal dimensions of less than 12 nm along at least one of threeorthogonal directions. Throughout this specification, the exemplarynanoparticles will be primarily disclosed and described as substantiallyspherical in shape.

Solid nanoparticles, which are also known as nanocrystals or quantumdots, have been formed from a wide variety of materials using a numberof different techniques which involve both top-down and bottom-upapproaches. Examples of the former include standard photolithographytechniques, dip-pen nanolithography, and focused ion-beam etching. Thelatter comprises techniques such as electrodeposition or electroplatingonto templated substrates, laser ablation of a suitable target,vapor-liquid-solid growth of nanowires, and growth of surfacenanostructures by thermal evaporation, sputtering, chemical vapordeposition (CVD), or molecular beam epitaxy (MBE) from suitable gasprecursors and/or solid sources.

Solid nanoparticles may also be formed using conventionalpowder-processing techniques such as comminution, grinding, or chemicalreactions. Examples of these processes include mechanical grinding in aball mill, atomization of molten metal forced through an orifice at highvelocity, centrifugal disintegration, sol-gel processing, andvaporization of a liquefied metal followed by supercooling in an inertgas stream. Nanoparticles synthesized by chemical routes may involvesolution-phase growth in which, as an example, sodium boron hydride,superhydride, hydrazine, or citrates may be used to reduce an aqueous ornonaqueous solution comprising salts of a non-noble metal and/or noblemetal. Alternatively, the salt mixtures may be reduced using H₂ gas attemperatures ranging from 150° C. to 1,000° C. These chemical reductivemethods can be used, for example, to make nanoparticles of nickel (Ni),cobalt (Co), iron (Fe), copper (Cu), tungsten (W), and/or theircombinations or alloys. Powder-processing techniques are advantageous inthat they are generally capable of producing large quantities ofnanometer-scale particles with desired size distributions.

In one embodiment, nanoparticle cores may be formed on a suitablesupport material by pulse electrodeposition. This method involvesinitially preparing a thin film of a carbon powder on a glassy carbonelectrode. Prior approaches have typically used a thin layer of Nafion,a polymer membrane, to affix the carbon powder onto the glassy carbonelectrode. However, in this embodiment Nafion is not needed since a thinfilm of carbon powder is formed directly onto the glassy carbonelectrode. A pH-buffered solution containing a salt of the metal to bereduced is then produced and the carbon-coated electrode is immersed inthe solution. Reduction of the metal itself is accomplished by applyinga first potential pulse to reduce the metal ions from solution andnucleate metal nanoparticles on the surfaces of the carbon powdersupport. This is followed by a second potential pulse whose duration isused to control the final size of the thus-formed nanoparticles.

The first potential pulse is thus used to control the nucleation ratewhereas the second potential pulse is used to drive subsequent growth ofthe nucleated nanoparticles. By using two separate potential pulses,both the number density and the size of nanoparticle cores produced canbe independently controlled by the duration of the pulses at the twopotentials. In one embodiment, the first potential may range from −0.5 Vto −0.2 V, while the second potential may range from −0.3 V to −0.1 V.In another embodiment the first potential may range from −1.6 V to −1.0V, whereas the second potential ranges from −0.9 V to −0.7 V. Allpotential pulses are typically measured versus a Ag/AgCl (3 M NaCl)reference electrode.

When forming nanoparticle cores from a solution containing non-noblemetal ions, the pH of the solution may be higher than 4 so that themetal nanoparticles formed after potential pulse deposition will bestable. A suitable non-noble metal solution to produce Ni or Conanoparticle cores may comprise 0.1 M to 0.5 M NiSO₄ or CoSO₄,respectively, with 0.5 M H₃BO₃. It is contemplated that other solublesalts of Ni may also be used. Pulse potential deposition of Ni or Conanoparticle cores may then proceed by applying a first potential pulsein the range of −1.6 V to −1.0 V, followed by a second potential pulsein the range of −0.9 V to −0.7 V. All potentials are typically measuredversus a Ag/AgCl (3 M NaCl) reference electrode with the pulse durationbeing adjusted to obtain the desired density and size distribution.

In another embodiment nanoparticle cores may be formed by adding achemical reducing agent to a solution comprising a salt of the desiredmetal. A typical reducing agent is NaBH₄ or N₂H₄ with NaOH or Na₂CO₃being added to adjust the pH of the solution. An exemplary solution thatmay be used to form Ni nanoparticle cores on a carbon support comprises10 mg carbon powder, 3 ml H₂O, and 1 ml 0.1 M NiSO₄ or NiCl₂. Prior toadding the reducing agent to reduce the Ni nanoparticles, the solutionmay be sonicated and deaerated. The reduction process proceeds by addinga small amount of the reducing agent to the slurry while vigorouslystirring the solution in a deaerated environment at room temperature toproduce Ni nanoparticles dispersed on a carbon powder support. In aparticular embodiment, the solution contains an excess of Ni ions toensure that the added reducing agent is fully consumed.

By using a small amount of a strong reducing agent to control theparticle size, the need for a surfactant is substantially reduced oreliminated. Furthermore, the process mimics pulse potential deposition,as described above, since the reaction initially occurs very rapidly andthen is abruptly terminated once the reducing agent has been fullyconsumed. Besides avoiding the use of a surfactant, consumption of allof the reducing agent allows subsequent processes to be performed in thesame solution. For example, a salt of a different metal may be added tothe reactor without needing to first filter out the thus-formednanoparticle cores and create a new solution. This is particularlyadvantageous when forming a shell layer by galvanic displacement since asalt of a noble metal can be added directly to the solution as describedin Section II below.

In yet another embodiment, nanoparticle cores may be formed by heating adry mixture of carbon and adsorbed first metal ions in hydrogen. Thecarbon may be in powder or nanotube form and may be functionalized byimmersing in HNO₃ and H₂SO₄ mixed acids, resulting in anion groups, suchas, —CO₂H and —SO₃H, being attached at the carbon surface. The exemplarydry mixture of carbon and the first metal ions is formed by stirring aslurry comprising a salt of first metal and functionalized carbon powderor carbon nanotubes for more than 10 hours, and then, filtering out theaqueous solution. After being dried at room temperature, the mixture isheated to about 700° C. in hydrogen for about 2 hours yieldingnanoparticles of the first metal on a carbon support. Before proceedingwith the production of the nitride stabilized nanoparticles, thecarbon-supported nanoparticle cores of the first metal may be cooled inliquid argon (Ar).

It is to be understood that the methods of forming the nanoparticlesdescribed above are merely exemplary. Alternative methods which arewell-known in the art and which are capable of forming nanoparticleswith the desired shape, size, and composition may be employed. The keyaspect is that the nanoparticles provide a template of a predeterminedsize onto which a shell layer can be deposited. In an embodiment, thesize of the nanoparticle cores is adjusted to maximize the catalyticactivity of the resulting nitride stabilized nanoparticles.

II. Nanoparticle Shell

Once nanoparticles having the desired shape, composition, and sizedistribution have been fabricated, the desired shell layer may then beformed. The particular process used to form the shell layer is notintended to be limited to any particular process, but is generallyintended to be such that it permits formation of films havingthicknesses in the monolayer-to-multilayer thickness range. It is to beunderstood, however, that while the process of preparing core-shellnanoparticles is described sequentially, the cores and the shells of thecore-shell nanoparticles can also be formed in parallel.

For purposes of this specification, a monolayer (ML) is formed when thesurface of a substrate, e.g., a nanoparticle, is fully covered by asingle, closely packed layer comprising adatoms of a second materialwhich forms a chemical or physical bond with atoms at the surface of thesubstrate. The surface is considered fully covered when substantiallyall available surface sites are occupied by the adatoms of the secondmaterial. The surface may be considered fully covered when more than 90%of all available surface sites are occupied by the adatoms of the secondmaterial, or when more than 95% of all available surface sites areoccupied by the adatoms of the second material. When more than about 90%of all available surface sites are occupied the shell is considered tobe continuous and nonporous. If less than 90% of the surface sites ofthe substrate are not completely occupied, then the surface coverage isconsidered to be submonolayer. However, if a second layer or subsequentlayers of the adsorbant are deposited onto the first layer, thenmultilayer surface coverages, e.g., bilayer, trilayer, etc., result andare considered continuous and nonporous.

The process for forming a shell layer by galvanic displacement occurswhen the nanoparticle cores are immersed into a solution comprising asalt of a noble metal. Since the salt is a noble metal salt and the corematerial is a non-noble metal or non-noble metal alloy, an irreversibleand spontaneous redox reaction occurs, in which core surface atoms areoxidized and replaced by the noble ions contained in solution. The ratioof the outer and inner diameters of the thus-formed nanoparticles can becontrolled by varying the concentration of the noble metal ions and theduration for which the cores are immersed in the noble metal saltsolution.

As an illustrative embodiment, nanoparticle cores of a non-noble metalsuch as Cu, Ni, or Fe may be initially produced using any of thetechniques described in the foregoing Section I. The use of galvanicdisplacement is, however, especially advantageous when combined withchemical synthesis routes for the production of nanoparticle cores.Galvanic displacement proceeds by introducing the nanoparticles to asolution comprising a salt of a noble metal such as, for example, Pt,Pd, Ir, Ru, Os, Au, or Re, by immersion in a solution comprising one ormore of K₂PtCl₄, PdCl₂, IrCl₃, RuCl₃, OsCl₃, HAuCl₃, or ReCl₃,respectively.

Using a Ni core and a Pt salt as an example, the galvanic replacement ofsurface Ni atoms by Pt occurs via the reaction Ni+Pt²⁺→Ni²⁺+Pt toproduce Ni-Pt core-shell nanoparticles. Replacement of Ni surface atomsby Pt results in a reduction of the size of the Ni nanoparticle core.The final thickness and surface coverage by the resulting noble metalshell layer can be controlled by varying process parameters, such as theconcentration of the noble metal salt and the duration of the immersionin solution. In practice, many Ni particles of less than 3 nm indiameter disappeared after immersion in solution, suggesting that theywere completely replaced by Pt, and that during the process the Pt atomswere deposited onto large nearby particles. This may have the effect ofincreasing the overall size distribution of the remaining Ni-Ptcore-shell nanoparticles. The dissolution of smaller Ni cores isactually beneficial because it is generally undesirable to have Niparticles having sizes of less than 3 nm which inevitably formed duringsynthesis of the Ni cores without using surfactants. Furthermore, theshell layer formed via galvanic displacement is not limited to a singlemetal, but may be formed as an alloy having several constituents to forma binary, ternary, quaternary, or quinary alloy. This may beaccomplished, for example, by including more than one noble metal saltin the solution.

An important aspect of shell formation via galvanic displacement is theinhibition of oxidation of and/or removal of any oxide formed on thesurfaces of the nanoparticle cores once they have been fabricated. Theformation of a surface oxide layer significantly inhibits the galvanicdisplacement process, by forming metal-oxygen bonds at nanoparticle coresurfaces. Thus, transfer into a solution comprising a metal salt tofacilitate galvanic displacement by a noble metal is preferablyaccomplished in the absence of oxygen.

In one embodiment, galvanic displacement is performed by immersing thenanoparticle cores in a solution comprising 0.05 mM to 5 mM K₂PtCl₄ toproduce a Pt shell layer. In another embodiment a Pd shell layer may beformed by immersing the nanoparticle cores in a solution comprising 0.05mM to 5 mM Pd(NH₃)₄Cl₂. In yet another embodiment a Pd/Au shell layermay be formed by immersing the particles cores in a solution comprising0.5 mM Pd(NH₃)₄Cl₂ and 0.025 mM HAuCl₃. In yet another two embodiments aRu and an Ir shell layers may be formed by immersing the particle coresin a solution comprising 1 mM RuCl₃ and IrCl₃, respectively. Theduration of exposure in each of these exemplary metal salts may becalculated to obtain the desired thickness of the shell layer.

In an embodiment, carbon-supported nanoparticle cores of a non-noblemetal such as Ni or Co are formed using the chemical reduction, dry heattreatment under hydrogen, or pulse potential deposition processes, asdescribed in Section I above. When pulse potential deposition is used,the nanoparticles are transferred to a solution comprising the desirednoble metal salt in the absence of oxygen. When forming non-noble metalnanoparticle cores using chemical reduction methods, the non-noble metalsalt is present in excess, such that the reduction reaction proceeds tocompletion and all of the reducing agent is consumed. This permitsaddition of the desired concentration of a noble metal salt directly tothe solution, thereby avoiding the need to filter out and rinse the corenanoparticles formed by chemical reduction methods. This is advantageousbecause it prevents exposure of the nanoparticle cores to the ambientatmosphere where a surface oxide may form.

III. Nitridation of the Core

Once core-shell nanoparticles having the desired shape, composition, andsize distribution have been fabricated, the nitrogen may then beintroduced into the core. The particular process used to introducenitrogen into the core is not intended to be limited to any particularprocess, but is generally intended to permit formation of a metalnitride within the core. It is to be understood, however, that while theprocess of preparing nitride stabilized core-shell nanoparticles isdescribed sequentially, the process of introducing nitrogen into thecore can also be done during the core formation, during the shellformation, or both.

The metal nitride formation within the core may be initiated by thermalannealing the core-shell nanoparticles for 1 to 20 hours, followed byexposing the core-shell nanoparticles to a nitrogen precursor atelevated temperatures and ambient pressure for a time sufficient to forma metal nitride. In one embodiment, the amount of metal nitride withinthe core is between 0.1 and 100% wt. %. In another embodiment, theamount of metal nitride within the core is at least 2% wt. %, or atleast 5% wt. %. In yet another embodiment, the amount of metal nitridewithin the core is between 10 and 70% wt. %, or between 20 and 50% wt.%. In an embodiment where the metal is nickel, the amount of nickelnitride (Ni₄N) within the core is between 20 and 50% wt. %.

The nitrogen source is not particularly limited and can be selected fromammonia (NH₃), dinitrogen (N₂), nitric oxide (NO), and hydrazine (N₂H₄).In certain embodiments, the nitrogen source is ammonia. The core-shellnanoparticles may be thermally annealed at about 200, 250, or 300° C. innitrogen (N₂) gas for about 1-5 hours, followed by thermal annealing at400, 450, 500, 550, or 600° C. in ammonia (NH₃) for about 1-10 hours.Although both dinitrogen and ammonia may be used in such process, it isbelieved that ammonia functions as a precursor of nitrogen in theformation of the metal nitride. The manufacturing process is simple andcost-effective, providing nitride stabilized nanoparticles with highercatalytic activities and improved durability in combination with minimalloading of precious materials compared to catalysts currently in use.

In another embodiment the disclosed nanoparticles are manufactured by amethod which involves preparing a mixture which includes an organicsolvent, salts of a noble metal and a non-noble transition metal, andoptionally a carbon powder. The mixture may be stirred, sonicated and/ordeaerated for a period of time, for example from about 1 minute to aboutan hour or more. The organic solvent may be removed from the mixture byevaporation to form dried powders. In one embodiment, the mixture wasdried completely using a rotary evaporator with a heating bath at 50° C.The dried powders may then be thermally annealed as described above toform a metal nitride core and a thin noble metal shell. In oneembodiment, the annealing is performed in ammonia (NH₃) for 2 hours at500° C. at an ambient pressure. The nitriding treatment for the catalystcan also be carried out using a high pressure nitriding system at 500°C. at a high pressure up to 10 MPa (1500 psi). Additionally, theannealed powders may be cooled down to room temperature under a NH₃ flowin a closed furnace. The manufacturing process is simple andcost-effective, providing nitride stabilized nanoparticles with stillhigher catalytic activities and improved durability in combination withminimal loading of precious materials as compared with catalystscurrently in use. Because the this synthesis procedure does not involvea chemical reduction process in an aqueous solution, the possibility ofoxidation of Ni cores is excluded, thereby leading to a higher activityfor the ORR.

The mixture may comprise any suitable organic solvent. Examples ofsuitable organic solvents include but are not limited to acetone,chloroform, benzene, cyclohexane, dichloromethane, ethanol, diethylether, ethyl acetate, hexane, methanol, toluene, xylene, mixtures of twoor more of these, and derivatives of one or more of these.

The mixture may further comprise any of the metal salts mentioned above.The solution may comprise a soluble salt of Ni and a soluble salt of Ptin an organic solution. The soluble salt of Ni may be, for example,Ni(acac)₂. The soluble salt of Pt may be, for example, Pt(acac)₂.Formation of the core/shell nanoparticles may be accomplished byannealing the dried powder at 400 to 700° C. under NH₃ to form a corecomprising nickel nitride Ni_(x)N (x=3 or 4). In yet another embodimentthe salt of the noble metal of the formation of shell may be Pd(acac)₂and is used in combination with a Ni salt to form nitride stabilizedNi-Pd core-shell nanoparticles. In yet another embodiment, the salt ofthe noble metal of the formation of shell may be Pd(acac)₂ andAu(CH₃)₂(acac) and is used in combination with a Ni salt to form nitridestabilized Ni-PdAu core-shell nanoparticles. In yet another embodimentthe salt of the noble metal of the formation of shell may be Ir(acac)₃and is used in combination with a Ni salt to form nitride stabilizedNi-Ir core-shell nanoparticles.

IV. Exemplary Embodiments

The nitride stabilized nanoparticles may have a continuous and nonporoussurface shell with a nitride metal core. The nitride metal core has astructure which induces lattice contraction and surface smoothening ofthe shell. The nitride stabilized nanoparticles may have an externaldiameter of less than 20 nm with a shell thickness of 0.5 nm to 3 nm,which is equivalent to a shell wall thickness of 2 to 12 atomic layers.In an embodiment, the nitride stabilized nanoparticles have an externaldiameter of 0.5 nm to 10 nm with a shell wall thickness of 2 to 4 atomiclayers (0.5 nm to 1 nm). In another embodiment the nitride stabilizednanoparticles have an external diameter of 3.5 nm and a shell wallthickness of 2 atomic layers (0.5 nm). The nitride stabilizednanoparticles preferably are single crystal, having a single latticeorientation across each nanoparticle.

An exemplary embodiment will be described in detail with reference toFIGS. 3-14. Another exemplary embodiment will be described in detailwith reference to FIGS. 15-20. In these embodiments, Ni nanoparticlesfabricated on carbon powder supports are used as the core material andPt is used as the shell material. The experimental data and the densityfunctional theory (DFT) calculations indicate that nitride has abifunctional effect that facilitates formation of the core-shellstructures and improves the performance of the Pt shell by inducing bothgeometric and electronic effects. Synthesis of inexpensive NiN coresopens up possibilities for designing various transition metal nitridebased core-shell nanoparticles for a wide range of applications inenergy conversion processes.

In summary, using various techniques the nitride stabilized core-shellstructure of the catalyst, which is stable against corrosion during ORR,was investigated. The Examples and Kuttiyiel, et al. (2012) Nano Lett.12:6266-6271, the contents of which are incorporated herein byreference, reveal that the high ORR activity and durability of PtNiNcatalyst is attributed to a Ni nitride core, modifying the behavior ofthe Pt shell by inducing both geometric and electronic effects. Theseadvances open up broad possibilities for the design and synthesis ofvarious transition metal nitride based core-shell nanoparticles for awide range of applications in energy conversion processes and devices.

EXAMPLE 1

Initially, the carbon supported platinum (Pt) and nickel (Ni)electrocatalyst was prepared by mixing 20.7 mg of K₂PtCl₄ and 27.7 mg ofNi(HCO₂)·2H₂O salts with 74.2 mg high area Vulcan XC72R (E-TEK) carbonblack in aqueous solution. While purging with Ar in an ultrasonic bathfor an hour, the salts were reduced by adding NaBH₄. The mixtureobtained was washed and rinsed with Millipore filtered water, and thendried. The dried sample was annealed at 250° C. under N₂ gas for 1 hourin a tube furnace which was followed by annealing under NH₃ gas for 2hours at 510° C. Finally the sample was cooled down to room temperatureunder NH₃ flow in closed furnace. The sample was then washed in 0.1 MH₂SO₄ to dissolve Ni precipitates on carbon support surfaces, and thenrinsed and dried again.

EXAMPLE 2

Physical characterization of the nanoparticles was performed by electronenergy-loss spectroscopy (EELS) mapping for Pt M (2122 eV) and Ni L (855eV) edges using a scanning transmission electron microscope (STEM)equipped with aberration-correction system (Hitachi; HD-2700C). Themicroscope is also equipped with a cold field emission electron sourceand a high resolution Gatan Enfina energy-loss spectrometer. The STEMimaging and EELS were performed using a 1.3 Å electron probe with aprobe current of about 50 pA. The STEM convergence angle is around 28mrad while the collection angle is from 114 to 608 mrad. In thisexperimental condition, the contrast of images directly related withatomic number (Z-contrast). The energy resolution for EELS is about 0.4eV estimated form the half-width of zero loss peaks. Thecarbon-supported nanoparticles were dispersed in water and thendeposited on a lacy-carbon TEM grid (EMS, Hatfield, Pa.).

The resulting nanoparticles had a nearly sphere-like shape as shown inFIG. 3a and FIG. 4b with an average diameter of 3.5 nm (˜2-7 nm) asshown in FIG. 3b . Overlapping the two-dimensional mapping of Pt and NiEELS signal from a single nanoparticle as shown in FIG. 4a (dottedlines) validates the core-shell structure. The EELS line scan profileindicates the distribution of Pt and Ni components in a representativesingle nanoparticle, where the Pt shell thickness can be directlymeasured and plotted as shown in FIG. 4c . The Pt shell thicknessmeasured on various nanoparticles was around 0.5 to 1.0 nm, equivalentto 2 to 4 monolayers of Pt on the Ni rich core. The overall weightpercentage of Pt and Ni in the carbon supported PtNiN core-shellnanoparticles was 10.2% and 3.5% respectively which accounts to a molarratio of 1, as determined by inductively coupled plasma optical emissionspectrometry (ICP-OES) measurements.

EXAMPLE 3

The core-shell nanoparticles were further characterized usingsynchrotron X-ray diffraction (XRD). The X-ray measurements wereperformed at the X7B beamline at National Synchrotron Light Source(NSLS), Brookhaven National Laboratory (BNL; Upton, N.Y.). Theinstrument parameters (Thompson-Cox-Hastings profile coefficients) werederived from the fit of a LaB₆ reference pattern. Two sets ofmeasurements were done: One with PtNi sample reduced in N₂ at 250° C.and the other was PtNiN sample annealed at 510° C. in NH₃The wavelengthof X-ray used was 0.3184 Å. XRD patterns were recorded on a Mar345 imageplate detector and the recording time for a spectrum was ca 2.6 min. Allthe XAS (x-ray absorption spectroscopy) measurements were carried out atthe NSLS, BNL using Beam Line X19A. The electrocatalyst was pressed andsealed in an electrochemical cell. The measurements were carried out atthe Pt L₃ edge (11,564 eV) and Ni K edge (8333 eV) in 1M HClO₄ atdifferent potentials in room temperature. The data were processed andanalyzed by Athena and Artemis software (Ravel, B.; Newville, M. JSynchrotron Radiat 2005, 12, 537-541).

The synchrotron X-ray diffraction (XRD) patterns suggest the formationof nickel-nitride (NiN) compound. The diffraction pattern of the asprepared core-shell nanoparticles shown in FIG. 5a exhibits only thereflections of Pt and Ni₄N that has the structure of primitive cubiclattice. The pattern points to the formation of a PtNi solid-solutionalloy with an average size of 3.6 nm estimated from the Scherer'sequation. Another feature is that peaks ascribed to reflections from(200) and (220) of the Ni metal phase are missing, indicating that thecatalyst contains mostly NiN phases. A relatively smaller peak appearingat 8.98° may correspond to a Ni₃N or Ni metal phase and both having(111) phase reflections at the 2θvalue.

EXAMPLE 4

To clarify the absence of Ni metal, the XRD pattern of the PtNinanoparticles annealed at 250° C. in N₂ was analyzed before NH₃treatment. PtNi forms a solid solution alloy when annealed at 250° C. inN₂ without showing any discrete peaks for Ni (see FIG. 6). The Ni₃N(111) phase appears only after the catalyst undergoes the NH₃ treatment.This suggests that some of the Ni₄N phases are decomposed to Ni₃Nphases, which might happen when the sample is cooled down to roomtemperature in an NH₃ environment, as Ni₃N is formed at temperaturesbetween 200 and 350° C. Moreover the inclusion of N in the PtNinanoparticles pushes the Pt (111) peak to higher angles, as shown inFIG. 6, and an increase line broadening was also observed indicating adistortion of the lattice due to N atoms. This change in the latticestructure is due to the chemical interaction of metal atoms with N,resulting in the formation of nitrides. Thus, synchrotron XRD confirmsthat Ni in the PtNiN core-shell nanoparticles is nitrided forming Ni₄Nspecies.

EXAMPLE 5

The stabilizing effect of the core-shell structure for PtNiNnanoparticles was determined by in situ X-ray absorption near edgespectroscopy (XANES) as shown in FIG. 5b . In situ XANES of the Ni Kedges from the PtNiN nanoparticles in 1 M HClO₄, together with referencematerial, viz., Ni foil (thickness ≈10 μm) suggest that electronicproperties of Ni have been changed by alloying with N and Pt. Also nochanges in energy were observed at various potential of 0.41-1.11 V,signifying that the Pt shell is protecting the Ni core from oxidation.The XANES data for Pt L₃ edges offer strong evidence of decreasedoxidation of Pt in PtNiN nanoparticles in comparison with commerciallyavailable Pt nanoparticles as shown in FIG. 5c and FIG. 8. The high Ptoxidation potential (lower extent of Pt oxidation) of PtNiN catalystsuggests the interaction of the underlying metal via geometric andelectronic effects. A decreased Pt oxidation can also be observed fromthe comparison of voltammetiy curves for PtNiN/C and Pt/C catalyst shownin FIG. 7.

EXAMPLE 6

The electrocatalytic activity of carbon supported PtNiN core-shellnanoparticles toward the ORR was benchmarked against the commerciallyavailable Pt/C catalyst (E-TEK, 10% wt. of 3.2 nm Pt nanoparticles onVulcan XC-72 carbon support). For both PtNiN/C catalyst and Pt/Ccatalyst, an aqueous dispersion (1 mg/mL) was prepared and sonicated for5 minutes. A thin film of the electrocatalyst was prepared on a glassycarbon rotating electrode (area: 0.196 cm²) for electrochemicalmeasurements using 15 μL of the dispersion. The electrodes were thencovered by a small amount of a Nafion solution (10μof 2 μg/5 μL) anddried in air before measurements. The loading amount of Pt for PtNiNcatalyst was 7.84 μg/cm² and for Pt/C catalyst was 7.65 μg/cm², based onthe geometric electrode area and the actual content of Pt measured byICP. Solutions were prepared from Optima perchloric acid obtained fromFisher and MilliQ UV-plus filtered water (Millipore). An Ag/AgCl/KCl(3M) electrode was used with a double-Junction chamber as a referenceand all potentials, E, are quoted with respect to reversible hydrogenelectrode (RHE). Cyclic Voltammetry (CV) characterization of thecatalysts in the absence of oxygen was typically carried out in thepotential range from 1.1 V to 0.0 V at a scan rate of 20 mV/s inAr-saturated 0.1M HClO₄ electrolyte.

The cyclic voltammetry (CV) curves for PtNiN catalysts shown in FIG. 7did not show any anodic currents ascribed to the oxidation/dissolutionof Ni, demonstrating that Ni is protected by the Pt shell. The half wavepotential measured from the ORR polarization curves at 1600 rpm (seeFIG. 9) for PtNiN core-shell catalyst was 905 mV, which was 55 mV higherthan pure Pt/C catalyst. The kinetic current was calculated from the ORRpolarization curves by using the Koutecky-Levich equation at various rpmfrom 225 to 2500 rpm in O₂-purged 0.1 M HClO₄ solution at a sweep rateof 10 mV/s. RDE cycling stability test for PtNiN catalyst were conductedin air saturated 0.1M HClO₄ in the potential range from 0.G to 1.05V atroom temperature. ORR polarization curves were recorded in O₂-saturatedelectrolyte at various rotation rates from 225 to 2500 rpm at a scanrate of 10 mV/s. RDE cycling stability test for PtNiN catalyst wereconducted in air saturated 0.1M HClO₄ in the potential range from 0.6 to1.05 V at room temperature. For the ORR at a RDE, the Koutechy-Levichequation can be described as follows:

$\frac{1}{i} = {\frac{1}{I_{k}} + \frac{1}{B\;\omega^{1/2}}}$where i is the experimentally measured current, I_(k) is the kineticcurrent, B is a constant and ω is the rotation rate. Using the dataobtained from the nanoparticles (see FIG. 7b ), the Koutecky-Levichplot, i.e., the inverse current (1/i) plotted as a function of theinverse of the square root of the rotation rate, (ω^(1/2)) is presentedin the inset figure of FIG. 7b . The kinetic currents for ORR can bedetermined from the intercepts of the 1/i axis at ω^(1/2)=0.

To calculate the mass activity, the kinetic current was normalized tothe loading amount of Pt. To compare specific activity, the current wasnormalized to the electrochemically active surface area (ECSA). The ECSAwas calculated by measuring the charge collected in the H_(upd)adsorption/desorption region after double-layer correction and assuminga value of 210 μC/cm² for the adsorption of a hydrogen monolayer. Atroom temperature, the PtNiN catalyst exhibited mass and specificactivities of 0.86 A/mg_(Pt) and 1.65 mA/cm², respectively at 0.9 Vversus a reversible hydrogen electrode (RHE), which were both around 4.5to 6.5 times greater than that of the Pt/C (0.20 A/mg_(Pt) and 0.24mA/cm²).

EXAMPLE 7

Accelerated durability tests were also performed by applying linearpotential sweeps between 0.6 and 1.05 V at 50 mV/s in air-saturated 0.1M HClO₄ solution at room temperature. After 35000 cycles, the CVmeasurements showed no loss in ECSA for the PtNiN core-shellelectrocatalyst (see FIG. 9b ). In contrast with Pt/C that has beendemonstrated to lose almost 45% of its initial area, the PtNiNcore-shell electrocatalyst had much better resilience under highoxidizing conditions. To measure the ORR degradation after cycling theinitial and final half wave potentials were compared at 1600 rpm in O₂saturated 0.1 M HClO₄ solution. After 35000 cycles the ORR measurementsshowed only 11 mV loss in its half wave potential, suggesting that thePtNiN catalyst has a very good stability for the ORR. STEM-EELS analysiswas used to examine the structure of the PtNiN core-shell nanoparticlesafter potential cycling to elucidate the questions about Ni leaching outin harsh acid environment (see FIG. 10 and FIG. 11). The overlapping ofthe Ni EELS and Pt signal of a single representative nanoparticle (FIG.10a and FIG. 10b ) after 35000 potential cycles clearly shows that thecore-shell structure of the PtNiN nanoparticle is intact furtherattesting the performance durability of the catalyst.

EXAMPLE 8

To elucidate the enhanced ORR activity and durability of PtNiN/Ccompared to those of Pt/C, density functional theory (DFT), calculationswere carried out using a sphere-like nanoparticle model with 1.7 nm asshown in FIG. 12. Specifically, the Vienna ab initi simulation package(VASP) code was applied for spin-polarized density functional theory(DFT) calculations. Only the Γ point for k sampling and a cut-off energyof 400 eV for nanoparticle calculations were used, while (3×3×3) wasused for the bulk calculation of Ni₄N. The projector augmented wavemethod (PAW) with the generalized gradient approximation (GGA) using therevised Perdew-Burke-Ernzerhof (RPBE) function was used. For thenanoparticle calculations, to simulate the core-shell nanoparticle(PtNi₄N; Pt is a shell and Ni₄N is a core) with a composition of 182 Pt,19 Ni, and 4 N atoms, a sphere-like nanoparticle model with ˜1.7 nm wasconstructed. As described, the core is a Ni₄N-like structure having acrystal lattice defined as a face centered cubic. All atoms were fullyrelaxed for optimizations.

In order to examine the ORR activity of the electrocatalysts, thebinding energy of atomic oxygen was calculated on the (111) plane of ananoparticle (BE-O). The binding energy is defined asBE-O=E[O-NP]-E[NP]-E[O],where E[O-NP], E[NP], and E[O], respectively, are the calculatedelectronic energies of an adsorbed oxygen-atom on a nanoparticle, aclean nanoparticle, and an oxygen atom (³O). In addition to the BE-Ocalculations, surface strains were estimated to examine geometriceffects according to (a−a_(o))/a_(o·). a and a_(o) are the averagedPt-Pt distance in the Pt monolayer and the calculated Pt-Pt distance ofbulk Pt (2.824 Å). In particular, to interpret the enhanced stability ofPtNiN/C, the diffusion of Pt atoms was examined from the second layer tothe defect sites on the topmost layer (i.e. the vertex site).

To simulate the experimental finding of the Ni₄N of the core materialshown in FIG. 5, the mole ratio of Ni and N was kept at approximately20% by using the bulk Ni₄N structure (a_(0,DFT)=3.756 Å) and to savecomputational time, only two Pt layers were used for modeling the Ptshell composed of 182 Pt, 19 Ni, and 4 N atoms (Pt_(2ML)Ni₄N). As shownin FIG. 13 and Table 1, the surface strain, the d-band center of Pt inthe shell, and the binding energy of oxygen (BE-O) were calculated as adescriptor for the ORR activity.

TABLE 1 Calculated binding energy of oxygen (BE-0), surface strain andd-band center (eV) for different structures. d-band center Surfacestrain BE-O (eV) (%) (eV) Pt −1.92 −3.04 −4.09 Pt_(1mL)Ni₄N −2.18 −4.46−3.63 Pt_(1mL)Ni −2.32 −5.60 −3.12 Pt_(2mL)Ni₄N −1.94 −3.19 −3.94Pt_(2mL)Ni −1.99 −3.36 −3.83

Similar to the previous study (Kuttiyiel, K. A.; Sasaki, K.; Choi, Y.;Su, D.; Liu, P.; Adzic, R. R. Energ Environ Sci 2012, 5, (1),5297-53048), an oxygen atom was placed at a fcc active site on thenanoparticle to calculate BE-Os (see FIG. 12). The results show thatPt_(2ML)Ni₄N and Pt_(2ML)Ni (two monolayers of the Pt shell on a Nicore) behave more like a pure Pt than Pt_(1ML)Ni (one monolayer of thePt shell on a Ni core) and Pt_(1ML)Ni₄N (one monolayer of the Pt shellon a Ni4N core). By adding one more layer of Pt in the shell, theeffects of Ni and Ni₄N cores on the Pt shell are significantly decreased(see FIG. 13). In addition, the surface contraction in the Pt shell isreduced by changing the core from Ni to Ni₄N, leading to an up-shiftedPt d-band center and the strengthened O—Pt interaction (BE-O, −3.83 eVfor Pt_(2ML)Ni and −3.94 eV for Pt_(2ML)Ni₄N). Yet, compared to pure Pt,a more contraction on the surface of Pt_(2ML)Ni₄N is observed (surfacestrain; −3.19% for Pt_(2ML)Ni₄N and −3.04%, for Pt, FIG. 14), whichleads the down-shifted d-band center of Pt (FIG. 13) and the weaker BE-O(BE-O; −3.94 eV for Pt_(2ML)Ni₄N and −4.09 eV for Pt, FIG. 14a ).Therefore, the DFT results support the experimental finding of thehigher ORR activity of PtNiN than Pt (FIG. 14b ). It is believed thattoo much Ni in the core leads to the instability of the nanoparticles,due to a too highly strained surface (FIG. 13 and Table 1). It may beevident that the Ni₃N phase is negligibly found in the X-ray spectrumcompared to the Ni₄N phase (FIG. 6). It is also well-known that PtNicore-shell electrocatalysts are not durable in acidic condition due tothe significant Ni dissolution. (Wang, C. et al. Adv. Funct. Mater.2011, 21 (1) 147-152).

EXAMPLE 9

To gain an understanding of the enhanced durability of PtNiN/C, in theabove-noted DFT study, the unavoidable imperfection of the nanoparticleswere taken into account. It has been found that vacancies are morefavorably formed at the vertex and edge sites than at terraces owing totheir lower formation energies. Thus only the diffusion of Pt from avertex of the inner shell to vacancy sites at the vertex of the outmostshell was considered. Using the Pt_(2ML) Ni₄N model, the calculationsshow that the energy cost for the Pt diffusion depends on the existenceof N atoms. For the inner Pt atom next to the N atom, it needs only 0.15eV, while for those far away from the N atom the needed energy is 0.33eV, which is consistent with that using Pt_(2ML)Ni (0.37 eV). Incontrast, pure Pt requires a much higher energy for the diffusion (0.42eV). Therefore, these calculations suggest that Pt_(2ML)Ni₄N has ahigher stability than Pt, by enabling the easier diffusion of Pt fromthe inner shells to the surfaces, filling the defect sites and thuspreventing the dissolution of Pt into the electrolyte. Accordingly, ahigher N concentration may facilitate the Pt diffusion and therefore,increase the durability of the nanoparticles.

To further examine this effect, one more N atom was added at theinterface between the Pt shell and the Ni₄N core, assuming the presenceof a localized Ni₃N, as shown in FIG. 5. By interacting with one more Natom, the energy cost for the inner Pt atom to diffuse to the vacancysite on the surface is decreased from 0.15 to 0.10 eV, confirming thatthe concentration of N is crucial to the durability of the catalysts.Overall, using Ni nitrides as the core enhances the ORR activity anddurability via both geometric and electronic effects. The nitriding ofthe Ni core tunes the electronic structure of the Pt shell to display ahigher ORR activity than Pt (electronic effect). Simultaneously, thepresence of N atoms in the core allows a facile diffusion of interactedPt from inner shells to the surface, filling the vacancy sites(geometric effect), resulting in augmented durability of the catalysts.

EXAMPLE 10

Carbon supported Pt and Ni electrocatalyst was prepared by mixing 331.5mg of Pt(acac)₂ and 220 mg of Ni(acac)₂ salts with 500 mg high areaVulcan XC72R carbon black in 100 ml of acetone. After mixing thesolution in an ultrasonic bath for an hour, the mixture was driedcompletely using a rotary evaporator with a heating bath (50° C.). Thedried sample was annealed for 2 hours at 500° C. under a NH₃ gas at anambient pressure. The nitriding treatment for the catalyst can also becarried out using a high pressure nitriding system at 500° C. at a highpressure up to 10 MPa (1500 psi). Finally the sample was cooled down toroom temperature under a NH₃ flow in a closed furnace. The particles aremonodispersed on the carbon supports, and have average diameters ofabout 3.5 nm as shown in FIGS. 15a-15c . The atomic structure of thecatalyst synthesized by this procedure is similar to the structures ofthe particles made in Example 1, having Ni₄N cores encapsulated by thinatomic Pt layers (about 2 to 4 atomic Pt layers) as shells.

The electrocatalytic activity and stability of the carbon supportedPtNiN core-shell nanoparticles of Example 10 toward the ORR were firsttested by RDE measurements. The results are shown in FIGS. 16-18 and inTable 2. The half wave potential (E_(1/2)) measured from the ORRpolarization curves at 1600 rpm for the PtNiN/C core-shell catalyst was932 mV, which is higher than the PtNiN/C catalyst of Example 1 (905 mV)and a commercial Pt/C catalyst (850 mV). The kinetic current wascalculated from the ORR polarization curves by using the Koutecky-Levichequation at various rpm in O₂-purged 0.1M HClO₄ solution at a sweep rateof 10 mV/s. To calculate the mass activity, the kinetic current wasnormalized to the loading amount of Pt and to compare specific activitythe current was normalized to the electrochemically active surface area(ECSA) calculated by measuring the charge collected in the H_(upd)adsorption/desorption region after double-layer correction. At roomtemperature, the PtNiN catalyst exhibited Pt mass activity (MA) andspecific activity (SA) of 0.93 A/mg and 3.86 mA/cm² at 0.9 V, which arehigher than those of the PtNiN/C catalyst of Example 1 (0.86 A/mg and1.65 mA/cm², respectively) and are also both around 4.6 to 16 timesgreater than those of the commercial Pt/C catalyst (0.20 A/mg_(Pt) and0.24 mA/cm², respectively). Accelerated durability tests were performedby applying linear potential sweeps between 0.6 and 1.0 V at 50 mV/s inair-saturated 0.1 M HClO₄ solution at room temperature. After 30,000cycles, the ORR and CV measurements showed very small losses in MA(−18%), SA (−7%), E_(1/2) (−13 mV), and ECSA (−13%), demonstrating thatthe PtNiN/C core-shell catalyst synthesized by this method has a verygood stability for ORR.

TABLE 2 Summary of MA, SA, E½, and ECSA results of carbon-supportedPtNiN nanoparticles. MA SA E_(1/2) ECSA d (A/mg) (mA/cm²) (mV) (m²/g) 00.91 3.86 932 89 10k 0.86 3.85 932 84 20k 0.78 3.70 928 79 30k 0.74 3.58919 77 (0/30k) (−18%) (−7%) (−13 mV) (−13%)

MEA tests of the PtNiN/C catalyst of Example 10 were performed at LosAlamos National Laboratory. FIGS. 19a and 19b show the H₂/airpolarization curves of PtNiN/C and commercial Pt/C catalysts at atemperature of 80° C. at a back pressure of 247 kPa. The Pt loadingswere 0.065 mg/cm² for PtNiN/C and 0.070 mg/cm² for Pt/C. Although the Ptloading for PtNiN/C is slightly lower than that of Pt/C, the overallperformance of PtNiN/C is higher than that of Pt/C. The MEA stabilitytests of PtNiN/C and Pt/C were also performed (FIGS. 20a and 20b ,respectively). The stability of PtNiN/C was fairly good; the activityeven increases after 10 k and 30 k cycles compared with the initial at 0k, which the commercial Pt/C catalyst show a significant degradation inactivity after 30 k cycles. Thus, it is demonstrated that the synthesismethod used for producing for these PtNiN core-shell catalyst ispromising to produce practical electrocatalysts not only for fuel cellbut also a wide range of other energy conversion systems.

V. Energy Conversion Devices

In a preferred application, the nitride stabilized nanoparticles may beused as an electrode in an energy conversion device such as a fuel cell.The use of the nitride stabilized nanoparticles advantageously providesminimal loading of precious metals, a heightened catalytic activity, andimproved durability. Use of the nitride stabilized nanoparticles in afuel cell is, however, merely exemplary and is being used to describe apossible implementation of embodiments of the present invention.Implementation as a fuel cell electrode is described, for example, inU.S. Pat. No. 7,691,780 to Adzic. It is to be understood that there aremany possible applications for the nitride stabilized nanoparticleswhich may include, but are not limited to, charge storage devices,applications which involve corrosive processes, as well as various othertypes of electrochemical or catalytic devices.

As shown in FIG. 21, a PEMFC that employs the nitride stabilizednanoparticles can, for example, use hydrogen gas (H₂) as a fuel source,which is introduced through a first electrode whereas an oxidant, suchas oxygen (O₂), is introduced through the second electrode. In oneexemplary configuration, the first electrode is the anode and the secondelectrode is the cathode. Preferably, the cathode is comprised of thenitride stabilized Pt nanoparticles. Under standard operating conditionselectrons and ions are separated from the fuel at the anode such thatthe electrons are transported through an external circuit and the ionspass through an electrolyte. At the cathode, the electrons and ionscombine with the oxidant to form a waste product which, in this example,is H₂O. The electrical current flowing through the external circuit canbe used as electrical energy to power conventional electronic devices.

The increase in the ORR attainable by incorporation of the nitridestabilized nanoparticles in one or more electrodes will produce anincrease in the overall energy conversion efficiency and durability ofthe fuel cell. Consequently, for a given quantity of fuel, a largeramount of electrical energy will be produced when using the nitridestabilized nanoparticle electrodes compared to conventional nanoparticleelectrodes. Furthermore, the increased durability provided by thenitride stabilized nanoparticle electrodes means that fuel cells whichincorporate such electrodes can be used for longer periods of timewithout a substantial drop in performance.

It will be appreciated by persons skilled in the art that theembodiments of the present invention are not limited to what has beenparticularly shown and described in the specification. Rather, the scopeof the present invention is defined by the claims which follow. Itshould further be understood that the above description is onlyrepresentative of illustrative examples of embodiments. For the reader'sconvenience, the above description has focused on a representativesample of possible embodiments, a sample that teaches the principles ofthe present invention. Other embodiments may result from a differentcombination of portions of different embodiments.

The specification has not attempted to exhaustively enumerate allpossible variations. That alternate embodiments may not have beenpresented for a specific portion of the invention, and may result from adifferent combination of described portions, or that other undescribedalternate embodiments may be available for a portion, is not to beconsidered a disclaimer of those alternate embodiments. It will beappreciated that many of those undescribed embodiments are within theliteral scope of the following claims, and others are equivalent.Furthermore, all references, publications, U.S. Patents, and U.S. PatentApplication Publications cited throughout this specification are herebyincorporated by reference in their entireties as if fully set forth inthis specification.

The invention claimed is:
 1. A method of forming nitride stabilizednanoparticles comprising, a non-noble metal solid solution core with asize of less than 100 nm along at least one of three orthogonal axes,and a continuous and nonporous noble metal shell surrounding thenon-noble metal solid solution core; wherein the non-noble metal solidsolution core comprises a nitride of a non-noble metal selected from thegroup consisting of nickel (Ni) nitride, cobalt (Co) nitride, iron (Fe)nitride, copper (Cu) nitride and mixtures thereof; formed by: preparinga mixture comprising an organic solvent, a salt of a first metal, and asalt of a second metal; evaporating the organic solvent from the mixtureto leave a dry powder; and annealing the dry powder under a nitrogensource to produce a nitride of the first metal.
 2. The method of claim 1wherein the nitrogen source comprises a nitrogen gas (N₂), an ammoniumgas (NH₃) or a combination thereof.
 3. The method of claim 1, whereinthe annealing comprises exposing the dry powder to a temperature frombetween about 400 and about 700° C. under NH₃ for 1 to 10 hours.
 4. Themethod of claim 1, wherein the mixture further comprises a carbonpowder.
 5. The method of claim 1, further comprising sonicating themixture before evaporating the organic solvent from the mixture.
 6. Themethod of claim 1, wherein first metal comprises at least one non-nobletransition metal.
 7. The method of claim 6, wherein at least onenon-noble transition metal comprises at least one of nickel (Ni), cobalt(Co), iron (Fe), copper (Cu), tungsten (W) or a combination thereof. 8.The method of claim 1, wherein the second metal comprises at least onenoble metal.
 9. The method of claim 8, wherein the at least one noblemetal comprises at least one of palladium (Pd), iridium (Ir), rhenium(Re), ruthenium (Ru), rhodium (Rh), osmium (Os), gold (Au), and platinum(Pt), or a combination thereof.
 10. The method of claim 1, wherein thefirst metal is nickel (Ni), and the second metal is platinum (Pt). 11.The method of claim 1, wherein the first metal is nickel (Ni), and thesecond metal is palladium (Pd).
 12. The method of claim 1, wherein thefirst metal is nickel (Ni), and the second metal is palladium (Ir). 13.The method of claim 1, wherein the salt of the first metal is Ni(acac)₂,and the salt of the second metal is Pt(acac)₂.
 14. The method of claim1, wherein the salt of the first metal is Ni(acac)₂, and the salt of thesecond metal is Pd(acac)₂.
 15. The method of claim 1, wherein the saltof the first metal is Ni(acac)₂, and the salt of the second metal isIr(acac)₃.
 16. The method of claim 1, wherein the mixture furthercomprises a salt of a third metal.
 17. The method of claim 16, whereinthe third metal is different from the first metal and the second metal,and comprises at least one of nickel (Ni), cobalt (Co), iron (Fe),copper (Cu), tungsten (W) palladium (Pd), iridium (Ir), rhenium (Re),ruthenium (Ru), rhodium (Rh), osmium (Os), gold (Au), or platinum (Pt).18. The method of claim 16, wherein the first metal is nickel (Ni), thesecond metal is palladium (Pd), and the third metal is gold (Au). 19.The method of claim 16, wherein the salt of the first metal isNi(acac)₂, the salt of the second metal is Pd(acac)₂, and the salt ofthe third metal is Au(CH₃)₂(acac).
 20. The method of claim 1, whereinthe organic solvent is at least one of acetone, chloroform, benzene,cyclohexane, dichloromethane, ethanol, diethyl ether, ethyl acetate,hexane, methanol, toluene, xylene, mixtures of two or more thereof, orderivatives of one or more thereof.